The present invention relates to an additive powder for coating materials or plastics, that is suited to be mixed in coating materials to impart corrosion resistance and weathering resistance, or to be mixed in plastics or rubbers to impart electromagnetic shielding properties or magnetic shielding properties, and a method of preparing the same.
Hitherto used as coating materials used in the field where corrosion resistance and weathering resistance are required are those mixed with powders comprising glass or metals. These powders can be grouped into (1) those in which the powder itself has corrosion resistance and the corrosion resistance can be imparted by covering a coating surface with the powder, as exemplified by glass powders and stainless steel powders, (2) those in which the powder serves as a sacrificial electrode to prevent corrosion of a coating surface, as exemplified by zinc powders, and (3) those which can improve weathering resistance by application of a metallic coating, as exemplified by aluminum powders.
Of these powders, used as the glass powders are those which resemble fragments of spheres obtained by breaking glass balloons.
Used as the stainless steel powders are those obtained by bringing a rolled sheet of stainless steel into intergranular corrosion followed by physical grinding, or those obtained by subjecting molten stainless steel to water-atomizing to form powder and physically forcing the powder to have fIat particles by means of a stamp mill or the like.
Used as the zinc powders are those obtained by heating and vaporizing a zinc metal in a retort, introducing the resulting zinc vapor into an air-cut-off condensor to effect rapid-cooling condensation, thus obtaining spherical powder, and physically forcing the resulting powder to have flat particles by means of a stamp mill or the like.
Used as the aluminum powders are those prepared by subjecting molten aluminum to gas-atomizing to obtain powder, and physically forcing the powder to have flat particles.
Recent years, a variety of amorphous alloys having good corrosion resistance have been also developed, and attempts have been made to impart better corrosion resistance by forming these amorphous alloys into powder and mixing the powder into a coating material.
As an example of such attempts, proposed in Japanese Unexamined Patent Publications No. 252668/1985 and No. 252669/1985 is to add in a coating material an amorphous alloy powder comprising scaly particles of several ten to several hundred .mu.m in length or width and not more than 5 .mu.m in thickness.
In general, those comprising leaf-shaped particles are considered to be preferred as these additive powders for coating materials. This is because, when the powders are mixed in a coating resin material and the mixture is coated by brushing or spraying, the powder particles are laid overlapping in parallel with the coating surface owing to the surface tension produced in the curing of the resin (this is called a leafing phenomenon) to form a continuous film comprised of the powder, thus protecting materials from the air and imparting good corrosion resistance and weathering resistance. However, even this leaf-shaped powder may cause the problem that the smoothness of a coating becomes poorer with increase in thickness of powder particles. Moreover, the leaf-shaped particle having an excessively long major axis tends to cause the cracking or peeling of the coating, resulting in a lowering of the strength of the coating.
While, the glass powders prepared by breaking glass balloons, which resemble fragments of spheres, can not give a perfect flatness, and hence can not cause the above leafing phenomenon in a good state, so that it has been impossible to impart sufficient corrosion resistance.
The stainless steel powders obtained by bringing a rolled sheet into intergranular corrosion followed by physical grinding or the stainless steel, zinc or aluminum atomized powders physically forced to have flat particles also tend to become irregular in their shapes, so that it has been impossible to obtain those comprising sufficiently thin and flat particles. For this reason, it has been still impossible to cause the above leafing phenomenon in a good state.
In the case of the above amorphous alloy powders described in Japanese Unexamined Patent Publications No. 252668/1985 and No. 252669/1985, limitations are made on the length, width and thickness in respect of the particle shape. However, in order to cause the leafing phenomenon in a good state and impart a good corrosion resistance, consideration should have been taken also in respect of a required minimum thickness, aspect ratio (ratio of the major axis to thickness), ratio of the major axis to the minor axis, or the like.
The above amorphous alloy powders are prepared by dropping from the upper part a powder previously formed into alloy by the method such as atomizing, bringing the powder to melt on the way it falls, with use of a heat source such as an acetylene flame, an oxygen-hydrogen flame or an oxygen-propane flame blown out from a nozzle concentrically disposed, and rapid-cooling the molten droplets simultaneously with the rolling by use of a twin-roll mill.
According to this method, however, it has been difficult to prepare a powder comprising particles having the shape defined in respect of the minimum thickness, aspect ratio (ratiO of the major axis to thickness), or ratio of the major axis to the minor axis. Moreover, since in this method the powder is brought to melt on the way it falls, the powder can not be fed in a large quantity, so that it has been impossible to produce the desired powder in a large quantity in an industrial scale.
Also known as methods of preparing a flaky powder with use of the amorphous alloy are a method in which hydrogen is adsorbed in a ribbon or strip prepared according to a conventional single roll method or cavitation method to make it brittle, followed by grinding by means of a stamp mill or ball mill and a method in which a melt is atomized on the roll surface in the single roll method to prepare a powder comprising particles of irregular shapes.
In these methods, however, the resulting powder has a particle thickness usually of from 5 to 20 .mu.m in approximation, and hence cracking may sometimes occur on the coating when the resulting powder is mixed in resin and used as a coating material, and thus could not been said to be suitable as a powder for coating materials.
On the other hand, in various plastics or rubbers, known are those in which a conductive filler is added in order to impart the action of electromagnetic shielding, or a reinforcing filler is added in order to reinforce plastics. Particularly used as the conductive filler is a metal filler comprising copper, nickel, silver or the like.
Widely used as these metal fillers are fibrous or dendritic fillers prepared by cutting metals according to a chatter vibrating method using a lathe turning machine
Since, however, almost all of these metal fillers are fibrous or dendritic, a number of gaps are formed between fillers when they are mixed into plastics or rubbers, showing the tendency that no sufficient effect can be obtained particularly in the case of metal fillers used for the purpose of electromagnetic shielding.